From the viewpoint of global environment protection, automobile lightening has strongly been desired for the purpose of making fuel-efficient automobiles. When a steel sheet is used for parts composing a vehicle, lightening has been attempted by applying a high-strength steel sheet and reducing the thickness of this steel sheet. On the other hand, to improve the collision safety of automobiles, further strengthening has been required for automobile parts, such as pillars, and there has been an increasing need for ultrahigh-strength steel sheets having higher tensile strength.
However, when thin steel sheets are made to have higher strength, the elongation EL or r value (Lankford value) thereof is lowered, resulting in the deterioration of press formability or shape fixability.
Under these circumstances, to realize high-strength structural parts for automobiles, a hot pressing method (a so-called “hot press method”) has been proposed (e.g., Patent Document 1), in which both press-forming and improving the strength of parts by hardening are achieved at the same time. This technique is a method in which a steel sheet is heated up to an austenite (γ) region not lower than an Ac3 transformation point thereof and then hot press-formed, during which the steel sheet is simultaneously hardened by being brought into contact with a press tool at ordinary temperature, to realize ultrahigh strengthening.
According to such a hot pressing method, the steel sheet is formed in a state of low strength, and therefore, the steel sheet exhibits decreased springback (favorable shape fixability), resulting in the achievement of a tensile strength in the 1500 MPa class by rapid cooling. In this regard, such a hot pressing method has been called with various names, in addition to a hot press method, such as a hot forming method, a hot stamping method, a hot stamp method, and a die quenching method.
FIG. 1 is a schematic explanatory view showing the structure of a press tool for carrying out hot press-forming as described above (hereinafter represented sometimes by “hot pressing”). In FIG. 1, reference numerals 1, 2, 3, and 4 represent a punch, a die, a blank holder, and a steel sheet (blank), respectively, and abbreviations BHF, rp, rd, and CL represent a blank holding force, a punch shoulder radius, a die shoulder radius, and a clearance between the punch and the die, respectively. In these parts, punch 1 and die 2 have passage 1a and passage 2a, respectively, formed in the inside thereof, through which passages a cooling medium (e.g., water) can be allowed to pass, and the press tool is made to have a structure so that these members can be cooled by allowing the cooling medium to pass through these passages.
When a steel sheet is hot pressed (e.g., subjected to hot deep drawing) with such a press tool, the forming is started in a state where a blank (steel sheet 4) is softened by heating to a temperature not lower than an Ac3 transformation point thereof. That is, steel sheet 4 is pushed into a cavity of die 2 (between the parts indicated by reference numerals 2 and 2 in FIG. 1) by punch 1 with steel sheet 4 in a high-temperature state being sandwiched between die 2 and blank holder 3 to form steel sheet 4 into a shape corresponding to the outer shape of punch 1 while reducing the outer diameter of steel sheet 4. In addition, heat is removed from steel sheet 4 to the press tool (punch 1 and die 2) by cooling punch 1 and die 2 in parallel with the forming, and the hardening of a material is carried out by further retaining and cooling steel sheet 4 at the lower dead point in the forming (the point of time when the punch head is positioned at the highest level: the state shown in FIG. 1). Formed products with high dimension accuracy and strength in the 1500 MPa class can be obtained by carrying out such a forming method. Furthermore, such a forming method results in that the volume of a pressing machine can be made smaller because a forming load can be reduced as compared with the case where parts in the same strength class are formed by cold pressing.
In the conventional hot pressing, a steel sheet is heated up to an austenitic region (e.g., about 900° C.) not lower than an Ac3 transformation point thereof, and the steel sheet is then cooled by a press tool for press-forming while being kept in a high-temperature state. Therefore, the steel sheet may easily have a temperature difference between its portion coming into contact with, and its portion not coming into contact with, the press tool composed of punch 1 and die 2, so that strain may be concentrated on its portion becoming relatively high temperature, or so that, for example, in deep drawing, a shrink flange becomes unshrinkable by cooling, both resulting in the deterioration of formability, and in particular, thereby making it difficult to achieve deep drawing.
In the hot pressing, a steel sheet is usually press-formed at about 700° C. to 900° C. and hardened in a press tool, and therefore, the steel sheet should be kept at the lower dead point in the forming (the point of time when the punch head is positioned at the highest level: the state shown in FIG. 1) for a certain period of time, resulting in the deterioration of productivity as compared with cold pressing.
For this reason, various techniques have hitherto been proposed even for increasing productivity. For example, Patent Document 2 discloses a technique of forming a steel sheet, while supplying a lubricant, with a press tool having a lubricant supply port so that a blank holding portion (blank holder 3 shown in FIG. 1) is easily shrunk and the steel sheet easily flows into a vertical wall portion (vertical wall portion of the press tool). However, this technique not only makes the structure of the press tool complicated, but also cannot solve a fundamental problem that the steel sheet easily has a temperature difference therein.
Patent Document 3 discloses a processing method of forming a steel sheet, while successively processing its portion becoming high temperature, and cooling its portion coming to have a smaller sheet thickness. However, even in this technique, the structure of a press toll is made complicated, and for example, in the case of deep drawing, it becomes difficult to maintain a shrink flange at high temperatures.
Patent Document 4 discloses a method of forming a steel sheet, while controlling the displacement of a blank holding portion depending on the sum of sheet thickness and clearance. This technique is effective in the case where a blank holder is a uniform shrink flange such as in cylindrical cup deep drawing. However, complicated forming results in the distribution of the location where wrinkle occurs and the location where wrinkle does not occur, and therefore, contact pressure is increased on crest and trough portions at the location where wrinkle occurs (peaks and bottoms of irregularities) and the temperature of their portions is more lowered, resulting in a distribution of strength in its entirety. As a result, the flowing of a blank into the vertical wall portion becomes unstable, instead resulting in the deterioration of deep drawability.
By the way, when a steel sheet is heated up to an austenite region (e.g., about 900° C.) not lower than an Ac3 transformation point thereof, the steel sheet is exposed to the air for several seconds when being moved from a heating oven to a press-forming machine, resulting in the formation of oxide layers (scales) on the surface of the steel sheet. The scales fall out in the press-forming, which become the cause for the formation of press marks and other defects. Furthermore, the presence of such scale marks deteriorates the application of a corrosion-resistant coating, and therefore, it becomes necessary to remove the scales by peening or any other treatment after the press-forming.
As an attempt to avoid the disadvantages caused by the formation of scales, surface-treated steel sheets, such as aluminized, galvanized, or galvannealed steel sheets, have been used as a material (blank) for press-forming, but this attempt has another disadvantage that surface treatment drives costs up and requires long time at the stage of heating (makes it impossible to heat the steel sheets rapidly in order to keep plated layers and achieve alloying). Furthermore, the formation of scales may also be avoided by controlling the atmosphere in a heating oven or around a press-forming machine, but this is not realistic because of its need for a large-sized apparatus.